Support XML and JSON inflector acronyms

Related to #361

Add special-case treatment of JSON and XML acronyms by declaring aliases
for the `JsonFormat` and `XmlFormat` constants.

In addition to the aliases, this commit also includes test coverage to
cover existing behavior.

The original PR included a comment citing that
`ActiveSupport::Inflector::Inflections#clear` was not working as
expected. It was resolved by [a76344f][], which was merged into `main`
prior to the `7.0.0` release. Since the CI matrix currently includes
`7-0-stable` as the minimum version, the code can rely on the resolved
behavior.

[a76344f]: rails/rails@a76344f

lib/active_resource/formats/json_format.rb

@@ -24,5 +24,7 @@ def decode(json)
         Formats.remove_root(ActiveSupport::JSON.decode(json))
       end
     end
+
+    JSONFormat = JsonFormat
   end
 end

lib/active_resource/formats/xml_format.rb

@@ -23,5 +23,7 @@ def decode(xml)
         Formats.remove_root(Hash.from_xml(xml))
       end
     end
+
+    XMLFormat = XmlFormat
   end
 end

test/cases/formats_test.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+
+class FormatsTest < ActiveSupport::TestCase
+  def test_json_format_uses_camelcase
+    assert_equal ActiveResource::Formats::JsonFormat, ActiveResource::Formats[:json]
+  end
+
+  def test_xml_format_uses_camelcase
+    assert_equal ActiveResource::Formats::XmlFormat, ActiveResource::Formats[:xml]
+  end
+
+  def test_custom_format_uses_camelcase
+    klass = Class.new
+    ActiveResource::Formats.const_set(:MsgpackFormat, klass)
+
+    assert_equal klass, ActiveResource::Formats[:msgpack]
+  ensure
+    ActiveResource::Formats.send(:remove_const, :MsgpackFormat)
+  end
+
+  def test_unknown_format_raises_not_found_error
+    assert_raises NameError, match: "uninitialized constant ActiveResource::Formats::MsgpackFormat" do
+      ActiveResource::Formats[:msgpack]
+    end
+  end
+
+  def test_json_format_uses_acronym_inflections
+    ActiveSupport::Inflector.inflections { |inflect| inflect.acronym "JSON" }
+
+    assert_equal ActiveResource::Formats::JsonFormat, ActiveResource::Formats[:json]
+  ensure
+    ActiveSupport::Inflector.inflections.clear
+  end
+
+  def test_xml_format_uses_acronym_inflections
+    ActiveSupport::Inflector.inflections { |inflect| inflect.acronym "XML" }
+
+    assert_equal ActiveResource::Formats::XmlFormat, ActiveResource::Formats[:xml]
+  ensure
+    ActiveSupport::Inflector.inflections.clear
+  end
+end